1. Field of the Invention
The present invention relates to seawater-based delayed cross-linking fracturing fluids and methods of using the fluids in fracturing subterranean formations penetrated by well bores.
2. Description of the Prior Art
Oil and gas wells often undergo hydraulic fracturing operations to increase the flow of oil and natural gas from subterranean formations. Hydraulic fracturing is accomplished by injecting a viscous fracturing fluid through the well bore into a subterranean formation to be fractured and applying sufficient fluid pressure on the formation to cause the production of one or more fractures therein. In offshore operations, the fracturing fluid can be prepared utilizing seawater to hydrate a gelling agent to form a viscous aqueous fluid. To promote adequate viscosity at increased well depths, cross-linking agents such as borate ion releasing compounds can be incorporated into the fracturing fluids.
Seawater-based borate cross-linked fracturing fluids perform satisfactorily in low temperature fracturing applications, typically about 200° F. or less. At these lower temperatures, the initial pH required to form a sufficiently cross-linked gel is about 9.5 or less. In general, a sufficiently cross-linked gel is defined as having a viscosity of about 200 centipoises or greater at 40/sec shear rate. In order to form a sufficiently cross-linked gel for use at formation temperatures in excess of 200° F., the initial pH of a borate cross-linked fracturing fluid must be greater than about 9.5. Elevation of the fracturing fluid pH to a level in excess of 9.5, however, poses several operational problems. For instance, seawater contains multivalent ions such as calcium and magnesium ions which form insoluble precipitates at a pH greater than about 9.5. The presence of solid precipitates reduces the proppant pack conductivity, and ultimately the productivity of the fracturing operation.
Furthermore, elevating the pH of the fracturing fluid to a pH greater than about 9.5 is difficult due to the formation of magnesium hydroxide. Hydroxyl ions needed to elevate the pH of the seawater are instead consumed in the formation of magnesium hydroxide. This reaction proceeds very slowly causing the pH change to be time-delayed and difficult to adjust. In addressing the problems associated with precipitate formation in high-temperature seawater-based fracturing fluids, prior art methods suggest the removal of solid precipitates by filtration. However, the proper disposal of magnesium hydroxide and other precipitates creates additional operational costs as well as environmental challenges.
In order to conduct fracturing operations at greater well depths, it is desirable to delay cross-linking of the fracturing fluid. In particular, a delayed cross-link is advantageous in the fracturing of offshore formations where such operations are usually performed at lower injection rates because of pumping equipment limitations. Reduced injection rates, typically about 10 barrels/minute or less, lead to increased pipe times. Pipe time refers to the time required for the fracturing fluid to make the transit from surface pumping equipment to the formation to be fractured. It is generally desirable to have cross-linking occur near the end of the pipe time as the fluid approaches the formation to be fractured. If cross-linking occurs too early, the increase in fracturing fluid viscosity will increase friction loss in the well bore and produce high pump pressures. To overcome these problems, the cross-linking of the fracturing fluid is delayed until the fluid approaches a location near or within the formation to be fractured.
For these reasons, there is a need for a seawater-based fracturing fluid that avoids precipitate formation and forms delayed cross-links in high temperature fracturing operations.